1. Field of the Invention
The present invention relates to a method for controlling an AC voltage to be applied to an electrifier used in an electrophotography type image forming apparatus to electrify a photosensitive body.
2. Related Backgrond Art
A convential method used to electrify a surface of an image bearing body such as, for example, a photosensitive body, a dielectric body or the like in an image forming apparatus such as, for example, an electrophotographic apparatus, an electrostatic recording apparatus or the like, is to perform corona electrification. Corona electrification electrifies a surface of the image bearing body without making contact by allowing a corona produced by applying a high voltage to a thin corona discharging wire to act on the surface of the image bearing body.
Increasingly, so as to use lower voltages, produce smaller levels of ozone, and to lower cost, a main current is being applied by a contact electrification method which electrifies a surface of an image bearing body by bringing a roller type, a blade type or a similar type of electrifying member into contact with the image bearing body and-applying a voltage to the electrifying member. In particular, the roller type electrifying member is capable of electrifying the surface of the image bearing body stably for a long time.
Though only a DC voltage may be applied to the electrifying member, this member can be uniformly electrified by applying an oscillating voltage so as to alternately cause discharge on a positive side and a negative side.
It is known that to average the electrification and uniformly electrify a body to be electrified one may, for example, apply an oscillating voltage which consists of an AC voltage having a peak to peak voltage not lower than twice a threshold value of a discharge start voltage (electrification start voltage) of a body to be electrified at a time of DC voltage application and a DC voltage (DC offset bias voltage) overlapped with the AC voltage.
A waveform of the oscillating voltage is not limited to a sine wave and may be a rectangular waveform, a triangular waveform of a pulse waveform. The oscillating voltage includes a voltage having the rectangular waveform which is shaped by periodically turning on and off a DC voltage, and a DC voltage whose value is periodically changed so as to have an output identical to that of an overlapped voltage consisting of an AC voltage and a DC voltage.
As described above, a contact electrification method which electrifies an electrifying member by applying an oscillating voltage to the electrifying member will hereinafter be referred to as xe2x80x9can AC electrification methodxe2x80x9d. Furthermore, a contact electrification method which electrifies an electrifying member by applying only a DC voltage will hereinafter be referred to as xe2x80x9ca DC electrification methodxe2x80x9d.
However, the AC electrification method allows a larger amount of electric charges to be discharged to an image bearing body than the DC electrification method, accelerates deterioration such as cutting of the image bearing body and may produce abnormal images such as a flowed (smeared) image in an environment where a temperature and a humidity are enhanced by discharge products.
In order to solve this problem, it is necessary to minimize the discharge which is caused alternately on the positive side and the negative side by applying a minimum required voltage.
However, a relation between a voltage and an amount of discharged electric charges is not always constant, but changes dependently on thicknesses of photosensitive layers and dielectric layers of image bearing bodies, environmental changes of electrifying members and air, etc. In an environment where a temperature and a humidity are low (L/L), materials are dry and resistance values are enhanced, whereby discharge is liable to hardly occur and a peak to peak voltage higher than a definite value is necessary to obtain uniform electrification, but when an electrifying operation is performed in an environment where a temperature and a humidity are high (H/H), materials absorb moisture and resistance values are lowered even at a lowest voltage value which allows uniform electrification in the L/L environment, whereby an electrifying member discharges electric charge in an amount larger than required. As a result, the electric charges which are discharged in such a large amount pose problems of image flow, production of blur, toner fusion, cutting of a toner bearing body due to deterioration of a surface of the image bearing body, shortening of a service life and so on.
In order to supress increases and decreases of discharged electric charges due to environment variations, there has been proposed xe2x80x9can AC constant current control methodxe2x80x9d which controls a value of a supplied AC current by applying an AC voltage to an electrifying member in addition to xe2x80x9can AC constant voltage control methodxe2x80x9d which always applies a constant AC voltage as described above. The AC constant current control method is capable of enhancing a peak to peak voltage value of an AC voltage in the L/L environment where resistance of materials is enhanced and lowering a peak to peak voltage value in the H/H environment where the resistance of the materials is lowered, thereby making it possible to supress the increases and decreases of the amount of discharged electric charges more effectively than the AC constant voltage control method.
An electrifying member may not always be in contact with a surface of an image bearing body but may instead be disposed proximate to the electrifying member with a void (gap), for example, of several tens of micrometers reserved (proximity disposition) as a discharge enabling area. The discharge enabling area is determined by a gap to gap voltage and a correction Paschen curve is securely maintained between the electrifying member and the image bearing body, and this proximity disposition is included within a range of the contact electrification in the present invention.
For further prolonging a service life of an image bearing body, however, even the AC constant current control method is not perfect in suppressing the increase and decrease of the amount of discharged electric charges which are caused by changes of resistance values due to manufacturing dispersion and contamination of an electrifying member, variations of an electrostatic capacity of an image bearing body after long use, dispersion of a high-voltage apparatus of an image forming apparatus main unit and so on. In order to suppress the increase and decrease of the amount of discharged electric charges, it is necessary to use devices for suppressing manufacturing dispersion of the electrifying member, environmental changes and deflections of a high voltage, thereby enhancing a manufacturing cost.
An object of the present invention is to provide an application voltage control method which is capable of allowing an electrifier to discharge electric charges in a constant amount.
Another object of the present invention is to provide an application voltage control method which prevents excessive electric charges from being discharged.
Still another object of the present invention is to provide a method to control a voltage to be applied to an electrifier comprising the steps of:
measuring an integral value of an AC current waveform; and
controlling an AC voltage so as to make the integral value constant in a predetermined time.
Further another object of the present invention is to provide a method to control a voltage to be applied to an electrifier comprising the steps of:
measuring an integral value of an AC current waveform:
detecting a peak value of the AC current waveform;
calculating an area of the AC current waveform from the detected peak value; and
controlling an AC voltage so as to obtain a definite difference between the integral value and the area.
Further objects of the present invention will become apparent from the following description